1. Field of the Invention
The present invention relates to an optical system or zoom lens system having a function for correcting an image blur caused by vibration of the optical system. More particularly, the present invention relates to an optical system, such as a zoom lens system or single-focal-length lens system, that is capable of preventing the blurring of an image due to vibration (hereinafter referred to as hand shakes) of the optical system such as occurs when shooting is performed with a camera held by hand.
2. Description of the Prior Art
Conventionally, typical causes of unsuccessful photographing have been hand shakes and improper focusing. However, since most cameras are nowadays equipped with an autofocus mechanism, and since the focusing accuracy of such autofocus mechanisms has been steadily improving, unsuccessful photographing is caused by improper focusing far less often now than before. On the other hand, as more and more cameras are equipped with a zoom lens, rather than with a single-focal-length lens, they are designed to have higher magnifications and to be more suitable for telephoto photography, and accordingly they are now more susceptible to hand shakes. As a result, it can safely be said that, today, unsuccessful photographing is caused exclusively by hand shakes. For this reason, a hand-shake correction function is indispensable in photographing optical systems.
As optical systems having a hand-shake correction function, such optical systems have been proposed in which part of their lens units are decentered for hand-shake correction. For example, U.S. Pat. No. 5,502,594 proposes a telephoto zoom lens system consisting of, from an object side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a negative refractive power, a fourth lens unit having a positive refractive power, and a fifth lens unit having a negative refractive power, wherein the third lens unit is decentered in a direction perpendicular to the optical system to achieve hand-shake correction.
In an optical system having a hand-shake correction function, it is required that the optical system offer satisfactory optical performance not only in its normal state (hereinafter also referred to as the pre-decentering state), but also in its hand-shake correction state (hereinafter also referred to as the post-decentering state), without causing unduly large aberrations (hereinafter also referred to as the decentering aberrations) as the result of the decentering of the lenses. However, the above-mentioned five-unit zoom lens system according to U.S. Pat. No. 5,502,594 is defective in that it does not offer satisfactory aberration characteristics in the hand-shake correction state (i.e. after decentering) when it corrects hand shakes of large angles. The hand-shake correction performance of this zoom lens system is evaluated, in its publication, with hand shakes of approximately 0.15.degree.. However, while it is often necessary to correct hand shakes of larger angles in actual shooting of night scenes or other with the camera held by hand, this zoom lens system inconveniently exhibits intolerably large aberrations with hand shakes of large angles.